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Equilibrium of forces

If the resultant force acting on an object is zero, the object is either stationary or moving at constant velocity. Such an object is in translational equilibrium.

If two forces act on a body, an object achieves translational equilibrium if and only if the forces are equal in magnitude and point in opposite directions.

If three forces act on a body, an object achieves translational equilibrium if and only if the vector sum of the three forces is zero (i.e. $$F_{1}+F_{2}+F_{3}=0$$). This means that the forces must form a closed triangle (i.e. a triangular loop).

Closed triangle created by three forces in equilibrium
Closed triangle created by three forces in equilibrium